As part of the continuing effort to improve the environmental quality, the Environmental Protection Agency has set certain standards for the quantity of particulates which can be emitted to the air from various industrial processes. The EPA has similarly specified the way in which compliance with their standards is to be measured. One such test for particulates is performed by weighing a filter paper of predetermined size, inserting it into a stack gas stream for a predetermined period and weighing it afterwards and calculating from the difference and the relative area of the filter and stack the total number of pounds of particulates emitted per hour. There are several difficulties with the EPA test as presently defined. One is that the stack gas stream typically contains materials such as H.sub.2 S0.sub.4 which are gaseous S0.sub.3 at higher temperatures, e.g., above about 400.degree. F., but tend to react with water vapor and to condense as acid when the stack gas temperature is below the dew point, which varies between about 225.degree. and 400.degree. F. depending on the concentration. Present EPA tests specify the temperature of the stack gas at which the sample is to be taken as being well below the dew point. Hence, liquified H.sub.2 S0.sub.4 tends to collect on the filter, interfering with measurements of the particulates, which are of different compositions. The test could therefore be improved as to accuracy by performing it in a region of the stack where the gases are well above the dew point, allowing the S0.sub.3 gases to pass through the filter, and not interfering with the accuracy of the measurement.
A second difficulty in the measurement of particulates in stack gas streams is that the filters, even when of very high quality quartz or borosilicate glass fiber filter materials, contain certain metallic impurities which tend to react with some of the components of the gas stream. Hence, some chemical species which would otherwise pass through the filter instead are attracted to the filter for chemical reaction and are bound up, thus again increasing the weight of the filter to a value higher than that which it would have had had only particulates physically trapped in the mesh of the filter been captured.